The present invention relates to audio speakers and particularly to compact loud speakers. In recent years, the number of applications to which compact speakers are put has grown substantially. This growth is partly due to the arrival of numerous new forms of consumer electronics and personal electronic music playing devices, many of which require or promote the use of accessory speakers for full volume delivery of high quality sound. The increased use of compact speakers has also been fueled by a general trend toward smaller bookshelf or desktop systems, rather than the cabinet work and larger speaker enclosures that had formed the benchmark for audio performance over many decades.
For many of these applications light weight and portability are important. For still others, cost is a major factor. For yet other applications, it may be desirable to optimize the performance of such a speaker in relation to a cabinet or other speaker housing. In such cases, detailed consideration must be given to the structure and acoustics both of the speaker and of the housing. However, the trend to small speakers poses numerous technical problems, especially at the lower frequency end of the spectrum, since a smaller diaphragm is less effective at radiating lower frequencies and, moreover, has a higher natural resonance. A full panoply of compensatory features, such as the use of higher drive current, longer throw coil constructions, more powerful magnet gap, improved diaphragm materials and new cabinet configurations may be needed to achieve the desired operation in a smaller size system.
Thus, it would be desirable to provide an improved compact speaker.
It would also be desirable to provide a housing in which the performance of a compact speaker is further enhanced.
It would also be desirable to devise such a speaker and housing, wherein the housing itself is adapted to be mounted in a cabinet, a wall space or other location as a unit, and to thereby adapt the mounting structure without extensive acoustic engineering or individualized design considerations.
One or more of these and other desirable ends are obtained with the present invention by a speaker wherein first and second annular magnets are arranged concentrically with each other and connected by a shunt structure at one end and a pole-defining structure at the other end to concentrate magnetic flux in a cylindrical gap. Like the magnets, the shunt and the pole structure are also annular, and these are stacked such that the combined magnetic assembly has an opening extending centrally therethrough. The voice coil of a speaker rides in the cylindrical magnetic flux gap and its drive leads may be brought out behind the speaker, through the central opening. In various embodiments the diaphragm of the speaker may communicate through the central opening with the volume of a tuned enclosure situated behind the speaker, thus allowing further control over total acoustics.
In accordance with one aspect of the invention, the annular magnets are axially poled and of opposite polarity, separated by a cylindrical magnet gap between the two magnets. Two shaped pole pieces, one lying against the upper face of each magnet, define a shallow voice coil gap of higher flux density substantially contiguous with the magnet gap. The construction may be applied to an assembly using two neodymium ring magnets, of 25 millimeter and 36 millimeter outer diameters, to achieve a total flux density over 1.4 Tesla in a one-inch voice coil gap with a total speaker weight below two ounces and a total energy of 100 milliWatt seconds in the gap. The mass of the costly neodymium is thus minimized while the available flux is efficiently focused in the gap and overall speaker performance excels. In particular, the magnet achieves this high energy in a very shallow gap, allowing the diaphragm to be strongly driven with small excursion. The central through opening facilitates lead handling, both during speaker assembly and during subsequent speaker installation. The opening may also be exploited to permits an effective level of either damped or resonant coupling to be achieved in a relatively shallow chamber. The chamber may be a ported enclosure that mounts in a flush or shallow panel or wall.